Earlier this month, Italian neurosurgeon Sergio Canavero made waves when he announced he’d discovered the key to performing a totally crazy surgery: transplanting a whole head onto a new body.

Canavero said he’d devised a way to fuse a severed head, presumably from someone with a life-threatening illness, onto a body donated by a brain-dead patient. He claims to already have patients lined up to test the treatment, which he estimates will be feasible within two years, according to the cover story of this week’s New Scientist.

This isn’t the first time scientists have experimented with head transplants.

In 1970, a neurological surgery professor at Case Western Medical School named Robert White successfully transplanted the head of a rhesus monkey onto the body of another one, whose head had been removed at exactly the same time. While the monkey lived and breathed for eight days, the spinal cord from the head and body were not fused, so it could not move. On the ninth day, the monkey died.

Philip Meyers, professor of neurological surgery at Columbia University, told BuzzFeed News via email that White would periodically appear on the evening news describing his new head transplant experiments. “The camera would show a quadriplegic monkey looking around the room, smacking its lips,” Meyers said. “The medical ethicists would go wild! Quite the spectacle.”

In 1999, White wrote that, “What has always been the stuff of science fiction — the Frankenstein legend, in which an entire human being is constructed by sewing various body parts together — will become a clinical reality early in the 21st century.”

Now Canavero is claiming that thanks to new medical tools that can help fuse together the bundles of nerves in the spinal cord in the neck and body, we may have finally reached that point.

There’s just one problem: He’s wrong.

“There’s no way he’s going to hook up somebody’s brain to someone’s spinal cord and have them be functional,” Dr. Chad Gordon, professor of plastic and reconstructive surgery and neurological surgery at Johns Hopkins University, told BuzzFeed News.

Here are just some of the reasons why a head transplant will not be coming to a hospital near you in two years.

Not all brains would survive the cold temperatures needed for the surgery.

Canavero proposes cooling the brain down so that, when the head is decapitated, blood flow would slow down enough to extend the amount of time it can survive without oxygen. A brain cooled down to hypothermic temperatures — roughly 12 to 15 degrees Celsius — can survive without oxygen for up to an hour instead of as little as four minutes.

But this isn’t a foolproof solution. “I don’t think anyone is going to tell you that they can completely eliminate all the hypothermia complications,” Dr. Christopher Winfree, assistant professor of neurological surgery at Columbia University, told BuzzFeed News. “If they do this on 10 patients, only three to four might survive.”

And, Winfree added, hypothermia is only the tip of the iceberg.

We can’t glue two ends of a spinal cord together.

In a paper published this month describing this hypothetical procedure, Canavero describes attaching the head to the donor body. The donor would have to be a brain-dead, living patient who had previously promised their body to science. Alongside the patient in the same surgical room, the donor would have his or her head decapitated, and the remaining body would then need to be fused to the recipient’s head.

Canavero suggests doing that with a compound called polyethylene glycol, which has been shown to promote nerve growth in the spinal cord and could be used as a sort of glue between the body and the head.

But “once you sever the spinal cord, there’s a series of things that prevent it from healing. A fancy glue is not going to fix that,” Winfree said. There’s a veritable obstacle course preventing its growth: Certain nerve cells will immediately form scars that will act as physical barriers to the two sides of the spinal cord connecting; other proteins and enzymes will be inhibiting their growth as well.

Even if you could get them physically close enough, nerves themselves don’t grow too much in the spinal cord. “So not only do nerve cells not want to grow, the external environment is telling them not to grow, and then there’s a physical barrier to them growing,” Winfree said. “So the polyethylene glycol is a bunch of hooey.”

Even if the glue worked, connecting millions of nerves together is not possible.

Gordon, who worked on the very first face transplant in the U.S. in 2008, said the complexity of the connections made in the spinal cord was just one of the big reasons why a head transplant couldn’t happen.

Adjoining the two ends of the spinal cord — which is a giant bundle of nerves stretching from the brain to the spine and branching out to all parts of our bodies — is probably impossible. “You’re talking about the epicenter of the central nervous system, something with millions of connections,” he said. “There’s no way.”

The spinal cord isn’t even the deal breaker.

Even if the brain and spinal cord didn’t fuse, that wouldn’t necessarily be a deal breaker. The patient would still be able to survive as a paraplegic. Other issues could prevent the head transplant patient from living altogether.

Even if the head and body were successfully fused, the newly attached body wouldn’t work.

Canavero’s paper goes on to describe how the patient would be kept in a coma for three to four weeks, during which time implanted electrodes would help stimulate nerve growth while the patient is kept completely still. However, that requires the body’s functions to be operating as usual — which they wouldn’t be.

The vagus nerve, at the base of the brain, controls bodily functions like heart rate, blood pressure, digestion, and more. Without this nerve, which could take about a year to grow back after it’s stitched together, a person would have to be on dozens of forms of artificial life support to survive. Otherwise the body’s most basic unconscious functions would all be going haywire.

“You couldn’t sustain a person for that long without a vagus nerve, unless you can somehow artificially support the whole body for about a year,” Winfree said. “I think that’s where this might break down.”

Even if the patient survived all of this, they would still never be able to walk.

Canavero’s claim that the head transplant patient would be able to walk within a year is perhaps the most outlandish fantasy.

“Walking is totally out of the question with our current spinal cord injury repair technology,” Winfree said. “That technology doesn’t exist — I’m not saying it never will, but certainly not now.”

Gordon agrees. Even the 2008 face transplant was a huge feat, and that was “probably about 1/1000th” the difficulty of hooking up the brain and the spinal cord.

“The nerves just did not function as we planned, and that was just a few of them,” Gordon said. “I also work on hand transplants — you can’t even get the hand to move right. Neurosurgeons just don’t know enough about the brain yet to even know what they need to connect. It’s hard to solve the problem if you don’t even know what that problem is.”

But, OK, it’s not totally impossible.

Despite all these roadblocks, head transplants shouldn’t be ruled out, experts say.

Gordon bets it will be at least a century until we see people with new bodies walking around. Winfree is slightly more optimistic:

“I’m not ruling anything out. A hundred years is a very long time. But what’s currently just an idea could eventually seriously benefit people with life-threatening illnesses destroying their bodies.”

Even Canavero admits, a successful surgery and recovery isn’t the end of the process. After their head transplant, as he wrote in 2013: “body image and identity issues will need to be addressed, as the patient gets used to seeing and using the new body.” An integral part of a successful head transplant, Canavero wrote, will be a hefty side of therapy.